1. Field of the Invention
The present invention relates to a non-sintered positive electrode for alkaline storage batteries and an alkaline storage battery using the same.
2. Description of Related Art
Recently, with the spread of portable apparatuses, alkaline storage batteries are strongly demanded to be high in capacity. Particularly, nickel-metal hydride storage batteries are secondary batteries comprising positive electrodes mainly composed of nickel hydroxide and negative electrodes mainly composed of a hydrogen-storing alloy, and these batteries have rapidly spread as secondary batteries of high capacity and reliability.
The positive electrodes for alkaline storage batteries will be explained.
The positive electrodes for alkaline storage batteries are roughly classified into sintered type and non-sintered type. The former are made by impregnating a nickel sintered substrate of about 80% in porosity obtained by sintering a core material such as punching metal and a nickel powder with a nickel salt solution such as an aqueous nickel nitrate solution and subsequently with an aqueous alkaline solution, thereby to produce nickel hydroxide in the porous nickel sintered substrate. In these positive electrodes, the porosity of the substrate is difficult to further increase, and amount of nickel hydroxide cannot be increased. Thus, there is a limit in enhancement of capacity.
As the latter non-sintered type positive electrodes, JP-A-50-36935 proposes those which comprise a foamed nickel substrate of about 95% in porosity comprising three-dimensionally communicating pores in which nickel hydroxide particles are held and which are being widely used as positive electrodes for alkaline storage batteries of high capacity. In these non-sintered positive electrodes, spherical nickel hydroxide particles having a high bulk density are used from the point of increase in capacity. As disclosed in JP-A-62-136761, particle size of the nickel hydroxide particles and size of the pores of the foamed nickel substrate are adjusted to proper values. Moreover, metallic ions such as cobalt, cadmium and zinc are usually dissolved in the nickel hydroxide particles in the state of solid solution for the improvement of discharge characteristics, charge acceptance and life characteristics.
Here, since the size of the pores of the foamed nickel substrate is set sufficiently larger than the particle size of nickel hydroxide, the charge and discharge reaction smoothly proceeds in the nickel hydroxide particles present near the substrate skeleton which maintains current collection, but the reaction does not sufficiently proceed in the nickel hydroxide particles apart from the skeleton. Therefore, in non-sintered positive electrodes, the nickel hydroxide particles are electrically connected to each other using a conductive agent to improve utilization ratio of the packed nickel hydroxide particles. In many cases, divalent cobalt oxides such as cobalt hydroxide and cobalt monoxide are used as the conductive agent. These divalent cobalt oxides per se have no electrical conductivity, but are electrochemically oxidized to xcex2-cobalt oxyhydroxide having conductivity in the initial charging in the battery, which functions as an electrically conductive network. Thanks to the presence of the conductive network, the utilization ratio of active material packed at high density can be greatly increased in the non-sintered positive electrodes and thus increase of capacity can be attained as compared with in the sintered positive electrodes.
However, even the non-sintered positive electrodes having the above construction are not complete in current collecting performance of the conductive network and have their upper limits in utilization ratio of nickel hydroxide particles. Furthermore, the above positive electrodes suffer from the problems that when the battery is overcharged or kept with being short-circuited or stored for a long period or at high temperatures, the capacity of the positive electrodes lowers by the subsequent charging and discharging. This is because the electrochemical oxidation reaction in the battery as mentioned above cannot completely change a bivalent cobalt oxide to xcex2-cobalt oxyhydroxide and besides the function of the conductive network is apt to deteriorate.
Recently, as a means for improving the incompleteness of the conductive network, JP-A-8-148145 and U.S. Pat. No. 5,629,111 disclose a method which comprises heat treating (oxidizing) cobalt hydroxide in the active material of the positive electrode in the presence of an aqueous alkaline solution and oxygen (air) outside the battery to modify the cobalt hydroxide to a cobalt oxide having a disordered crystal structure and an oxidation number higher than 2. Similarly, JP-A-9-147905 discloses improvement of cobalt oxides having a cobalt valence of 2.5-2.93, and JP-A-9-259888 discloses characteristics of a battery made using xcex2-cobalt oxyhydroxide prepared in the similar manner.
Moreover, the above-mentioned JP-A-8-148145 additionally mentions application of the similar heat treatment to nickel hydroxide solid solution particles having a coating layer of cobalt hydroxide (hereinafter referred to as xe2x80x9cCo(OH)2-coated Ni particlesxe2x80x9d). This process has an advantage that amount of cobalt used can be reduced because of improvement in dispersion of cobalt by previously preparing the Co(OH)2-coated Ni particles. On the other hand, as to the method of preparation in this case, JP-A-9-73900 discloses a method which comprises heating Co(OH)2-coated Ni particles containing an aqueous alkaline solution in a fluidized granulator under fluidization or dispersion. This treatment has the advantage that troubles such as formation of particle lumps due to agglomeration can be diminished.
The main object of the above-mentioned techniques published recently is basically that cobalt oxidation reaction which takes place at the initial charging of batteries is sufficiently performed outside the battery, since the reaction does not satisfactorily proceed under normal conditions. Accordingly, the defect caused by the incompleteness of the conductive network referred to above can be improved.
However, the above-mentioned cobalt oxide cannot be said to be complete in oxidation state and further improvement is required.
The inventors have perceived the above points and conducted detailed experiments and analyses, and, as a result, have found that characteristics of an active material for positive electrodes which comprises xcex3-cobalt oxyhydroxide having a cobalt valence higher than 3.0 can be further improved as compared with other active materials, but the positive electrode using the above active material shows a greater reduction in capacity upon repetition of charging and discharging cycles at high temperatures than conventional positive electrodes.
The reduction in capacity of batteries upon repetition of charging and discharging cycles at high temperatures is a phenomenon recognized also in conventional positive electrodes. In the case of conventional positive electrodes in which a bivalent cobalt oxide is added to a foamed nickel substrate as a conductive agent and the conductive network is formed by charging (oxidation) in the battery, the bivalent cobalt oxide is dissolved in an electrolyte and re-precipitated (production of cobalt complex ion and re-precipitation as cobalt hydroxide) during the period of from filling the electrolyte to the initial charging. Therefore, the nickel hydroxide solid solution particles are bonded to the foamed nickel substrate through cobalt hydroxide, and when oxidation of cobalt hydroxide takes place by the initial charging and the cobalt hydroxide changes to xcex2-cobalt oxyhydroxide or the like which does not dissolve in the electrolyte, the active material particles (which mean here the sum of the nickel hydroxide solid solution particles and the cobalt oxide conductive agent) are strongly bound to the foamed nickel substrate. However, when the charging and discharging cycles are repeated at high temperatures, the cobalt oxide which forms the conductive network gradually changes in structure to a thermodynamically stable oxide such as Co3O4 or CoHO2 and results in growth of crystal. The conductivity of the thus produced Co3O4 or CoHO2 is lower than in the initial state containing xcex2-cobalt oxyhydroxide, and hence the capacity of positive electrode lowers.
On the other hand, when a cobalt oxide which has been oxidized to the state of being higher than 3.0 in cobalt valence outside the battery is used as a conductive agent, cobalt hydroxide in the powdery state before production of a positive electrode is oxidized, and hence there is no strong binding between the active material particles and the foamed nickel substrate in the positive electrode produced using the above cobalt oxide. However, since the conductivity of the cobalt oxide higher than 3.0 in valence is very high at the initial charging and discharging cycle of the battery just after produced, the above problem can be ignored and the positive electrode yields a capacity higher than conventional positive electrodes. On the other hand, if charging and discharging cycles are repeated at high temperatures, the similar structural change of the cobalt oxide also occurs in the above positive electrode. In this case, the lack of the binding between the active material particles and the substrate causes a severe problem.
That is, in the case of a positive electrode using a cobalt oxide which has been oxidized, outside the battery, to the state of being higher than 3.0 in cobalt valence as a conductive agent, when charging and discharging cycles are repeated at high temperatures, reduction in capacity of the positive electrode occurs due to decrease in conductivity of the cobalt oxide per se which forms the conductive network (the same mode of reduction in capacity of conventional positive electrodes) and additionally the incompleteness of current collectivity caused by the lack of the binding between the active material particles and the substrate (the mode of reduction in capacity peculiar in this case). Accordingly, degree of the reduction in capacity is very high as compared with conventional positive electrodes.
The main object of the present invention is to improve characteristics of active materials for positive electrode by using xcex3-cobalt oxyhydroxide and solve the problem of the reduction in capacity of positive electrode caused by charging and discharging cycle at high temperatures in alkaline storage batteries.
The present invention which solves the above problem relates to a non-sintered positive electrode for alkaline storage batteries, characterized in that spherical nickel hydroxide solid solution particles having an average particle size of 5-20 xcexcm and a cobalt oxide conductive agent having an average particle size of 1 xcexcm or less and mainly composed of xcex3-cobalt oxyhydroxide having a cobalt valence of higher than 3.0 are held in a foamed nickel substrate having the number of pores of 80-160 pores/inch (PPI2D) and a thickness of skeleton of 30-60 xcexcm and the total occupying ratio of the spherical nickel hydroxide solid solution particles and the cobalt oxide conductive agent based on the whole electrode plate is in the range of 75-85 vol %, and further relates to an alkaline storage battery using the said positive electrode.
By employing the above construction, the lack of binding between the active material particles and the foamed nickel substrate which is peculiar to a positive electrode using a cobalt oxide of higher valence is sufficiently compensated by the intimate physical contact between the active material particles and the substrate (securement of current collection), and hence the reduction in capacity of positive electrode caused by charging and discharging cycle at high temperatures can be highly inhibited. That is, there can be provided an alkaline storage battery which is high in energy density and excellent in overdischarge resistance and besides excellent in cycle life characteristics at high temperatures. Other objects of the present invention will become apparent from the following detailed description.